Throw exception instead of segfaulting when passing a scalar to get_items.

[psobot]

Before this change:

In [10]: i.get_items(1)
[1]    97271 segmentation fault  ipython

After this change:

In [5]: i.get_items(1)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-6cfae0b544de> in <module>
----> 1 i.get_items(1)

ValueError: get_items accepts a list of indices and returns a list of vectors

[yurymalkov]

Hi @psobot. Thanks for the PR! It seems that one of the tests is failing. Can you please look at it?

[psobot]

Done, thanks! I forgot to account for the fact that it's currently valid to pass an n-dimensional array to get_items, which will then be flattened before returning results. (Not sure if that's intentional behaviour, but I've left it in, as some of the tests depend on that.)

[yurymalkov]

@psobot Thank you!